This invention relates to a coin discriminating apparatus adapted for use with an automatic vendingmachine, automatic money changing machine, automatic accounting machine and the like.
In one type of coin discriminating apparatus known in the art, the external dimensions of a coin are measured to discriminate between different types of coins. In another type, the weight of a coin is measured to distinguish between different types of coins.
The former type has the disadvantage of not being able to discriminate between a genuine coin in circulation and a false coin which has the same external dimensions. The latter type has a similar disadvantage.
In still another type of coin discriminating apparatus known in the art, discrimination between coins is effected by measuring both external dimensions and weight. This type permits discrimination of coins to be carried out with a slightly higher degree of precision. However, this type has the disadvantages of being complex in construction and lower in efficiency than the aforementioned two types. Moreover, this type is unable to discriminate between a genuine coin in circulation and a false coin which is made such that it has the same external dimensions and weight as the genuine coin.
Generally, an object placed in a free space has the properties of being a medium of vibration, and the rate at which acoustical vibrations are transmitted through the medium in the form of vertical waves or transverse waves can be expressed as a function of density, Young's modulus and Poisson's number which are inherent properties of the object. Density is a physical quantity which may vary depending on external dimensions and weight. On the other hand, Young's modulus and Poisson's number are physical quantities which are concerned with the material of the object when sound waves are transmitted.
Measurements of specific resonance frequency of a coin which may vary depending on its physical qualities show that, since a particular coin in circulation has external dimensions which are predetermined and its material has a density which is constant, the nature frequency of such coin is constant and that the coin has sharp resonance characteristics when excited at its natural frequency. When a coin is subjected to impacts in free space, it vibrates in various modes and produces sound waves characteristic thereof which include its natural frequency. Since natural frequency is determined by the external dimensions and weight of the coin and the density of its material, it is possible to discriminate between two types of coins with a high degree of precision by measuring the natural frequency of each coin.
In using this system for testing a coin to find out whether it is genuine or false or what type of coin it is, the coin to be tested must be made to vibrate correctly at its natural frequency. To this end, the coin to be tested can be excited by a speaker, its vibration can be sensed by a pressure sensitive senser, and selective oscillation can be initiated by means of a feedback loop. In this type of coin discriminating apparatus, it is required to arrange the exciter and the receiver in a manner such that they are disposed specially in close proximity to each other. This arrangement generally causes howling to take place, so that it is not desirable to use a device for detecting sound waves which does not rely on contacts as is the case with the combination of a speaker and a microphone.
Proposals have been made to use a system wherein a coin is brought into engagement with a contact member so that the latter may detect the vibration of the former as a mechanical vibration, and the mechanical vibration is received by a pressure sensitive senser made of barium titanate or other dielectric material which converts it into an electric signal. When the contact member affixed to the senser is not made by a suitable material, a noise will be produced between the contact member and the coin when the latter is brought into engagement with the former. Thus, the noise is added to the natural frequency of the coin, making it difficult to discriminate between one type of coin and another.
Also, if the vibration of a coin is reversed in phase with respect to the acoustic frequency of the speaker, the vibration of the coin will be damped and no vibration of the coin will take place at all. This phenomenon will occur between coins of the same type when there are differences betwen them in the dimensions, shape, weight and other physical quantities of coins.